1. Field of the Invention
The present invention relates to compositions and methods for drilling, cementing and casing boreholes in subterranean formations, particularly hydrocarbon bearing formations. More particularly, the present invention relates to oil or synthetic fluid based invert emulsion drilling fluids which combine high ecological compatibility with good stability and performance properties. Most particularly, the invention relates to clay-free invert emulsion drilling fluids.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. For the drilling fluid to perform its functions, its optimum chemical and rheological properties must be controlled.
Oil or synthetic fluid-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”)) holes, but may be used in other holes penetrating a subterranean formation as well. Unless indicated otherwise, the terms “oil mud” or “oil-based mud or drilling fluid” shall be understood to include synthetic oils or other synthetic fluids as well as natural or traditional oils, and such oils shall be understood to comprise invert emulsions.
Oil-based muds used in drilling typically comprise: a base oil (or synthetic fluid) comprising the external phase of an invert emulsion; a saline, aqueous solution (typically a solution comprising about 30% calcium chloride) comprising the internal phase of the invert emulsion; emulsifiers at the interface of the internal and external phases; and other agents or additives for suspension, weight or density, oil-wetting, fluid loss or filtration control, and rheology control. Such additives commonly include organophilic clays and organophilic lignites. See H. C. H. Darley and George R. Gray, Composition and Properties of Drilling and Completion Fluids 66-67, 561-562 (5th ed. 1988). An oil-based or invert emulsion-based drilling fluid may commonly comprise between about 50:50 to about 95:5 by volume oil or oleaginous phase to water or aqueous phase.
Recent technology as described for example in U.S. Pat. Nos. 7,462,580 and 7,488,704 to Kirsner, et al., introduced “clay-free” invert emulsion-based drilling fluids, which offer significant advantages over drilling fluids containing organophilic clays. As used herein, the term “clay-free” (or “clayless”) means a drilling fluid made without addition of any organophilic clays or lignites to the drilling fluid composition.
When used in drilling, “clay-free” invert emulsion drilling fluids have shown reduced downhole losses, reduced pressure surges and spikes, and less barite sag than traditional drilling fluids containing organophilic clay and lignites. Hole drilling is faster with “clay-free” invert emulsion drilling fluids and reservoir productivity is often greater.
A limiting factor in drilling a particular portion of a well is the mud weight (density of the drilling fluid) that can be used. If too high a mud weight is used, fractures are created in the formation with resulting loss of drilling fluid and other operating problems. If too low a mud weight is used, formation fluids can encroach into the well, borehole collapse may occur due to insufficient hydrostatic support, and in extreme cases safety can be compromised due to the possibility of a well blowout. Many times, wells are drilled through weak or lost-circulation-prone zones prior to reaching a potential producing zone, requiring use of a low mud weight drilling fluid and installation of sequential casing strings to protect weaker zones above the potential producing zone. A particularly critical drilling scenario is one that combines deepwater and shallow overburden, as is typical of ultra deepwater fields in Brazil. This scenario is characterized by high pore fluid pressure, low effective stresses, low fracturing gradients and narrow mud weight windows.
Commercially available clay-free invert emulsion drilling fluids may have less than preferred rheology at low mud weights, that is, mud weights ranging from about 9 ppg to about 12 ppg, with temperatures up to about 375° F. or higher. Addition of inert solids may improve the rheology, but result in a decreased rate of penetration during drilling and loss of or decline in other benefits seen with a clay free system. Such inert solids include for example, fine sized calcium carbonate, and the term as used herein is not meant to be understood to include or refer to drill cuttings. Low mud weight or reduced density clay-free oil based invert emulsion drilling fluids also may show a decline in the desired “fragile gel” strength characteristic of clay-free invert emulsion drilling fluids. “Fragile gel” strength generally refers to the ability of the drilling fluid to both suspend drill cuttings at rest and show a lack of a pressure spike upon resumption of drilling. Solids added to an invert emulsion drilling fluid may be difficult to remove later in the drilling process and can ultimately lead to poor control over the rheology of the drilling fluid system and decreased rate of penetration (ROP).
Mineral oils are often used in formulating invert emulsion fluids, and such oils include n-paraffins, mixtures of n-paraffins, iso-paraffins, and cyclic and branched chain alkanes. These base oils offer environmental compatibility and low mud weights. However, they also have low viscosities and invert emulsion fluids prepared with them typically need additives to impart adequate rheology.
Invert emulsion-based muds or drilling fluids (also called invert drilling muds or invert muds or fluids) comprise a key segment of the drilling fluids industry, but they are increasingly being subjected to greater environmental restrictions and performance and cost demands. The complexities and unpredictability of the interaction and behavior of the fluid components with each other and with the conditions encountered during drilling makes meeting these demands challenging. There is a continuing need and thus ongoing industry-wide interest in new drilling fluids that provide improved performance while still affording environmental and economical acceptance.